Fractionation column control



United States Pater FRACTIONATION COLUMN CONTROL Lucien H. Vautrain, Sweeny, Tex., assigner to Phillips Petroleum Company, a corporation of Delaware Application December 14, 1953, Serial No. 398,076

7 Claims. (Cl. 2.02-160) This'invention relates to control systems for fractionaton columns. In one specific aspect it relates to a control system for maintaining a predetermined Constant pressure on a fraction column.

Generally, fractionation comprises a series of boilings and condensations which are performed to separate a feed stream into two or more product streams by means of the difference in vapor pressure or boiling points of the constituents being separated. At each step, the vapor leaving the boiling liquid contains more of the material with the lower boiling point than does the remaining liquid. Furthermore, as this vapor is condensed the liquid that condenses rst is richer in vthe material with the highest boiling point. A fractionation column normally comprises a series of bubble trays which are placed one above the other. These trays are designed so that vapors lfrom af lower tray pass through the liquid in the next tray above. This action condenses a portion of the heavier Vmaterials in the vapors, and at the same time vaporizes a portion of the lighter liquid on the tray. In this manner, each tray acts as a reboiler for one distillation unit and as the condenser for the preceding unit. The liquid level on each tray is maintained by a weir placed at one edge thereof. When the liquid level on a tray increases above the weir the liquid overflows to the next tray therebeneath. The Ipressure on each tray is maintained for a given throughput by the depth that the bubble cap is submerged below the surface of the liquid and by the friction of the'vapors through the bubble caps. The temperature at each tray is maintained by the composition of material on the tray because each tray is at its boiling point, and the equilibrium temperature of the liquid at its boiling point is dependent -upon the pressure. Thus,

-by controlling the pressure at one point in the column the design of the column will regulate the pressure through- `out the column.

Nearly all fractionation control systems are based in part upon maintaining a constant pressure on the column.

VIn the control system of the present invention, the vapors removed from the top of the fractionation column are 1 passed through a cooler to a reflux accumulator. A portion of the liquid withdrawn from the reflux accumulator is passed back into the column as reflux and the remainder-constitutes the overhead product stream. A second by-pass line is connected between the top of the frac- Vtionator and the reflux accumulator, and separate valves y tain a predetermined constant pressure on a fractionation column.

Various other objects, advantages and features of this invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure l is a schematic view of the fractionation column control system of the present invention; and

Figure 2 is a more detailed view of the valve control system of Figure l.

Referring now to the drawing in detail and to Figure l in particular, there is shown a fractionation column 10 which is supplied with an input feed stream by a line 11 which enters an intermediate section of column 10. This feed stream is maintained at a predetermined rate by means of a flow rate controller 12 which adjusts a valve 13 in line 11 in response to the pressure differential across an orifice 14 in line 11 upstream from valve 13. Heat is supplied to column l() by a steam coil 16 disposed near the bottom thereof. Steam is passed into coil 16 at a constant rate through a line 17, which constant rate is maintained by a iiow rate controller 18 which adjusts a-valve 19 in line 17 in response to the pressure differential across an orice 20 which is disposed in line 17 upstream from valve 19. The spent steam from coil 16 is removed throughan outlet line 21.

A vapor stream is removed from the top` of column 10 through a line 22 which passes through a valve 23 and a cooler 24 into a reflux accumulator 25. A second by-pass line 26 having a valve 27 disposed therein com,- municates between line 22 and accumulator 25. The condensed vapor in accumulator 25 is removed therefrom through a line 28 having a pump Z9 therein. A portion of the liquid pumped through line 28 passes through a re-uX line 311 back into the upper portion of column 10. A constant rate of ow is maintained through line 30 by a ow rate controller 31 which adjusts a valve 32 in line 3i) in response to the pressure differential across orifice 33 disposed in line 3o upstream from valve 32. The remainder of the liquid pumped through line 28 passes through an overhead product line 35. The rate of flow through line 35 is maintained at a value proportional to the level of liquid in accumulator 25 by means of a liquid level controller 36 which adjusts a valve 37 in line 35 in response to the liquid level in accumulator 25. Accumulator 25 is provided with a vent line 4t) having a valve 41 therein. Valve 41 is regulated by a pressure recorder-controller 42 which is actuated by the pressure in accumulator 25.

The bottom product stream from column 10 is removed through a line 44 having a pump 4S therein. The rate of withdrawal of this product stream is a function of the level of liquid in the bottom of column 10. The flow rate through line 44 is regulated by a liquid level controller 46 which adjusts a valve 47 in line 44 in response to the liquid level in the bottom of column 10.

Valves 23 and 27 are operated by a pressure recordercontroller 50 which is actuated by the pressure on the top of column 10. These two valves act in opposition to one another such that valve 23 is moved toward an open position and valve 24 is moved toward a closed position when the pressure on the top of column 10 increases. Conversely, valve 27 is moved toward :an open position and valve 23 is moved toward a closed position when the pressure on the top of column 10 decreases. One particular embodiment of this control system is shown in greater detail in Figure 2. Pressure recordercontroller 50 can be a conventional commercially available instrument which operates to regulate an output air pressure in response to a pressure under measurement. Thus, instrument 50 is provided with a first conduit 51 which communicates with the interior of column 10 to measure the pressure on the top of column 10. A source of input air at a predetermined pressure is applied to instrument 50 through a line 52. The pressure transmitted by conduit 51 operates a valve in instrument 50 whereby a regulated output air pressure is transmitted by means of a line 53 to valves 23 and 27. Normally, more vapor is transmitted through cooler 24 than bypass line 26, such that valve 23 is larger than valve 27. Valve 23, therefore, preferably comprises a diaphragm operated butterfly valve, and valve 27 preferably comprises a diaphragm operated balanced poppet valve. Valve 23 is adjusted such that an increase in pressure applied thereto through line 53 rotates vane 55 in a direction to further open the valve, and valve 27 is adjusted such that an increase in air pressure applied thereto through line 53 tends to decrease the flow through line 26 by further seating the two valve heads 57 and 58. Conversely, a decrease in pressure in line 53 tends to close valve 23 and open valve 27.

The operation of the control system of this invention should now become apparent. It is desired that controller 50 function to maintain a predetermined constant pressure on the top of column 10. If for any reason the pressure in column should tend to increase, valve 23 is opened further and valve 27 is closed further to direct :a greater portion of the overhead vapor from column 10 through cooler 24. The condensation of this vapor in cooler 24 results in a pressure drop thereacross which tends to decrease the pressure on column 10. Conversely, if the pressure should tend to decrease on column 10, valve 27 is opened further and valve 23 is closed further. This allows a greater portion of the vapor to by-pass cooler 24 so that the pressure in accumulator 25 increases to increase the pressure on column 10. If yany non-condensible vapors are present in the overhead stream removed from column 10, it is important to provide a vent line 40 on the accumulator. This vent line is actuated by the pressure Within the accumulator whereby valve 41 is opened when the pressure rises above a control point and closes when the pressure falls below such a point. This control point is as much less than the controlled pressure on the column as is necessary to provide sufficient pressure drop to move the column overhead vapor through the overhead vapor line 22 into cooler 24 at the greatest rate ever desired.

As a specific example of one particular application of the control system of this invention, reference is made to a particular separation wherein the feedstream passed to column 10 contains debutanized gasoline. This stream is passed into column 10 at a rate of approximately 300 barrels per hour. Approximately 225 barrels per hour of liquid are removed through bottom line 44 with 75 barrels per hour of overhead product being removed through line 35. The reflux passed back to column 10 through line 30 is at a rate of 'approximately 175 barrels per hour. The temperature at the bottom of column 10 is approximately 255 F., the temperature at the 'top of column 10 is approximately 162 F. and the temperature in accumulator 25 is approximately 150 F. The pressure on the top of column 10 is maintained at thirty-five pounds per square inch gauge whereas the pressure at the bottom of column 10 is thirty-six pounds per square inch gauge and the pressure in accumulator 25 is twenty-five pounds per square inch gauge. The column normally is operated to partially depentanize the feed stream so that the overhead product stream contains essentially all of the isopentane present in the feed stream. Valve 23 is a 10 butterfly valve and valve 27 is a 4" poppet valve.

It should readily be apparent that the control system of this invention provides an accurate adjustment of the quantity of vapor passed through cooler 24. By controlling both the rate vapor is passed through: cooler 24 and the rate vapor is passed through by-pass line 26 an extremely sensitive pressure control system is provided. This is decidedly superior to the pressure control systems employed heretofore. It will be apparent that reflux accumulator pressure and fractionator column pressure are controlled by a single instrument. Furthermore, the reflux accumulator pressure can be maintained close to the column pressure to reduce the pumping capacity required to return reflux to the column. While the control system of this invention has been described in conjunction with a present preferred embodiment thereof, it is to be understood that the invention is not limited thereto.

What is claimed is:

l. In a fractionation system including a fractionation column, a cooler, a reflux accumulator, and first conduit means communicating between said column and said accumulator through said cooler; a control system to maintain a predetermined pressure in said column comprising, in combination, a first valve in said first conduit, second conduit means communicating between said column and said accumulator, a second valve in said second conduit means, pressure responsive means communicating with said column, and means under control of said pressure responsive means to open and close said first and second valves in unison but in opposition in response to pressure variations in said column.

2. In a fractionation system including a fractionation column, a cooler, a reflux accumulator, and first conduit means communicating between said column and said accumulator through said cooler; a control system to maintain a predetermined pressure in said column comprising, in combination, a first throttle valve in said first conduit means between said column and said cooler, second conduit means communicating between said accumulator and said first conduit means between said column and said first valve, `a second throttle valve in said second conduit means, pressure responsive means communicating with said column, and means under control of said pressure responsive means to open said first valve and close said second valve in unison when the pressure in said column increases and to close said first valve and open said second valve in unison when the pressure in said column decreases, thereby to maintain a predetermined pressure in said column.

3. In a fractionation system including a fractionation column, a cooler, a reflux accumulator, and first conduit means communicating between said column and said accumulator through said cooler; a control system to maintain a predetermined pressure in said column comrising, in combination, a pneumatically operated butterfly valve in said first conduit means between said column and said cooler, second conduit means communicating between said accumulator and said first conduit means between said column and said butterfly valve, a pneumatically operated poppet valve in said second conduit means, pressure responsive means communicating with said column to establish a pneumatic pressure proportional to the pressure in said column, and means to apply said pneumatic pressure to both of said valves whereby said butterfly valve is opened and said poppet valve is closed when the pressure in said column increases and said buttefly valve is closed and said poppet valve is opened when the pressure in said column decreases, thereby to maintain a predetermined pressure in said column.

4. In a fractionation system including a fractionation column, a first conduit communicating with an intermediate section of said column to supply a feed stream to said column, a cooler, a reflux accumulator, a second conduit communicating between the upper end of said column and said accumulator through said cooler, a third conduit communicating between said accumulator and a section of said column near the top of said column to supply reflux to said column, a fourth conduit communicating with said accumulator to deliver an overhead product stream from said column, a fifth conduit communicating with the bottom of said column to deliver a bottoms product stream from said column, and means for supplying heat to the lower portion of said column; a

control system for said fractionation system comprising, in combination, a iiow rate controller to maintain a predetermined rate of flow through said first conduit, a liquid level controller communicating with the lower section of said column to regulate the flow through said fifth conduit to maintain a predetermined liquid level in said column, a flow rate controller to maintain a predetermined rate of flow through said third conduit, a liquid level controller communicating with said accumulator to regulate the flow through said fourth conduit to maintain a predetermined liquid level in said accumulator, means to control the heat supplied to the lower section of said column to maintain a constant rate of heat addition to said column, a first Valve in said second conduit, conduit means communicating between said column and said accumulator, a second valve in said conduit means, pressure responsive means communicating with said column, and means under control of said pressure responsive means to open and close said first and second valves in unison but in opposition in response to pressure variations in said column.

5. In a fractionation system including a fractionation column, a first conduit communicating with an intermediate section of said column to supply a feed stream to said column, a cooler, a reflux accumulator, a second conduit communicating between the upper end of said column and said accumulator through said cooler, a third conduit communicating between said accumulator and a section of said column near the top of said column to supply reflux to said column, a fourth conduit communicating with said accumulator to deliver an overhead product stream from said system, a fifth conduit communicating with the bottom of said column to deliver a bottoms product stream from said column, and means for supplying heat to the lower portion of said column; a control system for said fractionation system comprising, in combination, a ow rate controller to maintain a predetermined rate of flow through said first conduit, a liquid level controller communicating with the lower section of said column to regulate the ow through said fth conduit to maintain a predetermined liquid level in said column, a flow rate controller to maintain a predetermined rate of flow through said third conduit, a liquid level controller communicating with said accumulator to regulate the ow through said fourth conduit to maintain a predetermined liquid level in said accumulator, means to control the heat supplied to the lower section of said column to maintain a constant rate of heat addition to said column, a pneumatically operated butterfly valve in said second conduit between said column and said condenser, a sixth conduit communicating between said accumulator and said second conduit between said column and said butter-A fly valve, a pneumatically operated poppet valve in said sixth conduit, pressure responsive means communicating with said column to establish a pneumatic pressure which is a function of the pressure in said column, and means to apply said pneumatic pressure to both of said valves whereby said butterfly valve is opened and said poppet valve is closed when the pressure in said column increases and said butterfly valve is closed and said poppet valve is opened when the pressure in said column decreases, thereby to maintain a predetermined pressure in said column and a maximum pressure in said condenser and accumulator.

6. In a fractionation system including a fractionation column, a cooler, a reflux accumulator, and first means for providing communication between said column and said accumulator through said cooler; a control system to maintain a predetermined pressure in said column comprising, in combination, a first valve in said rst means, second means for providing communication between said column and said accumulator, a second valve in said second means, a pressure responsive controller, means communicating column pressure to the responsive element of said controller, and means under control of said controller to further open said first valve and to further close said second valve in unison therewith when the pressure in said column increases and to further close said first valve and to further open said second valve in unison therewith when the pressure in said column decreases, thereby to maintain a predetermined pressure in said column.

7. In a fractionation system including a fractionation column, a cooler, a reflux accumulator, and first conduit means communicating between said column and said accumulator through said cooler; a control system to maintain a predetermined pressure in said column comprising, in combination, a pneumatically operated butterfly valve in said iirst conduit means between said column and said cooler, second conduit means communicating between said accumulator and said first conduit means between said column and said butterfly valve, a pneumatically operated poppet valve in said second conduit means, a source of pneumatic fluid, a third conduit means for conducting said pneumatic liuid to said butterfly and said poppet valves from said source, a pressure operated means disposed in said third conduit means between said source and said valves, a fourth conduit connected between the column and said pressure operated means for communicating column pressure to said pressure operated means thereby to regulate the pressure of the pneumatic iiuid supplied to said valves in proportion to the column pressure, whereby said butterfly valve is further opened and said poppet valve is further closed when the pressure in said column increases and said butterfly valve is further closed and said poppet valve is further opened when the pressure in said column decreases, thereby to maintain a predetermined pressure in said column.

References Cited in the file of this patent UNITED STATES PATENTS 2,380,745 Frantz July 31, 1945 2,445,255 Younkin July 13, 1948 2,456,398 Gerhold Dec. 14, 1948 2,622,565 Venus Dec. 23, 1952 2,709,678 Berger May 31, 1955 2,711,992 Kerner June 28, 1955 OTHER REFERENCES Instruments and Process Control, published by N. Y. State Vocational and Practical Arts. Association, 1945, pages 179-181.

Petroleum Refiner, vol. 27, No. 11, pages 594 and 595.

Oil and Gas Journal, November 25, 1948, pages 87, 89, 124 and 127.

Oil and Gas Journal, December 9, 1948, pages 75, 76 and 81. 

1. IN A FRACTIONATION SYSTEM INCLUDING A FRACTIONATION COLUMN, A COOLER, A REFLUX ACCUMULATOR, AND THE FIRST CONDUIT MEANS COMMUNNICATING BETWEEN SAID COLUMN AND SAID ACCUMULATOR THROUGH SAID COOLER; A CONTROL SYSTEM TO MAINTAIN A PREDETERMINED PRESSURE IN SAID COLUMN COMPRISING, IN COMBINATION, A FIRST VALVE IN SAID FIRST CONDUITSECOND CONDUIT MEANS COMMUNICATING BETWEEN SAID COLUMN AND SAID ACCUMULTOR, A SECOND VALVE IN SAID SECOND CONDUIT MEANS, PRESSURE RESPONSIVE MEANS COMMUNICATING WITH SAID COLUMN, AND MEANS UNDER CONTROL OF SAID PRESSURE RESPONSIVE MEANS TO OPEN AND CLOSE SAID FIRST AND SECOND VALVES IN UNISION BUT IN OPPOSITION IN RESPONSE TO PRESSURE VARIATIONS IN SAID COLUMN. 